1. Field of the Invention
The present invention relates to a damping force adjusting structure of a hydraulic shock absorber.
2. Description of the Related Art
In a damping force adjusting structure of a hydraulic shock absorber, there may be a structure provided with a cylinder which is filled with an oil liquid, a piston which is slidably fitted into the cylinder, and a piston rod in which one end is coupled to the piston and the other end is extended to an outer portion of the cylinder. A main disc valve may also be provided which controls a flow of the oil liquid generated by a sliding motion of the piston so as to generate a damping force. A back pressure chamber applies an internal pressure in a valve closing direction of the main disc valve, introducing a part of the oil liquid to the back pressure chamber, and controlling a valve opening of the main disc valve, as described in Japanese Patent Application Laid-Open No. 2005-344734 (patent document 1). In this damping force adjusting structure, a pilot oil path introducing the oil liquid to the back pressure chamber is pieced in the main disc valve, and a sub valve directly opening and closing the pilot oil path is provided in a back surface side of the main disc valve. Since the internal pressure of the back pressure chamber is not increased when the sub valve doses the pilot oil path, the valve opening pressure of the main disc valve becomes lower, and the damping force becomes lower. When the sub valve opens the pilot oil path, the internal pressure of the back pressure chamber is increased, the valve opening pressure of the main disc valve becomes higher, and the damping force becomes higher.
Further, in this damping force adjusting structure, there is provided a leak path leaking the pressure within the back pressure chamber upon reaching a predetermined pressure, and a normally closed relief valve is provided in the leak path. Accordingly, if the pressure in the back pressure chamber reaches the predetermined pressure, the relief valve is opened so as to leak the pressure in the back pressure chamber, thereby preventing an excessive ascent of the valve opening pressure of the main disc valve, that is, the damping force.
In the damping force adjusting structure described in patent document 1, the pilot oil path pieced in the main disc valve is set to a back pressure introduction path to the back pressure chamber, and the back pressure introduction path is opened and closed by the sub disc valve. The pressure of the back pressure chamber is set on the basis of a valve rigidity of the sub disc valve and a magnitude of the pilot oil path of the main disc valve, and it is therefore difficult to control the pressure in the back pressure chamber.
Further, the normally closed relief valve is provided in the leak path leaking the pressure within the back pressure chamber. A leak amount of the pressure within the back pressure chamber is set on the basis of the valve rigidity of the relief valve, and it is therefore difficult to control the pressure in the back pressure chamber.